To show blood vessels there are currently recording options available with which three-dimensional image data records can be created. Imaging modalities can be used for this purpose, e.g. computer tomography (CT), magnetic resonance tomography (MR) or 3D rotation angiography.
Important applications in such case are the diagnosis of vessel diseases such as aneurysms and stenoses and the planning of therapies. The planning is a matter of predicting the probability of a rupture of aneurysms and of selecting the therapy depending on this prediction or the selection of suitable therapeutic systems (e.g. stents) and their dimensions (e.g. diameter and length). Specifically in the assessment of rupture probability of aneurysms a three-dimensional image data set of the vessel structure can be used as the basis for a computer simulation (using “Computational Fluid Dynamics”), with which the probability of a rupture can be computed. An important variable here is for example the diameter of the neck of an aneurysm. The exact assessment of the aneurysm neck can under some circumstances influence the decision as to whether the latter is to be removed by a clinical intervention or whether what is referred to intravascular coiling is to be performed instead.
It is thus very important, as a basis for therapy planning, to have as exact as possible a three-dimensional image of the vessel, especially in the area surrounding the diseased vessel.
The three-dimensional image data sets generated with currently used imaging modalities do not however possess any high local resolution. With MR the signal-to-noise ratio is the limiting factor which restricts the resolution of the MR image to around 1 mm3. An x-ray image does in principle have a high local resolution; however a large part of local resolution is lost through the reconstruction of many x-ray images into one three-dimensional image data set, e.g. in CT or in 3D rotation angiography. To minimize the x-ray dose for the patient no resolutions of significantly more than 0.2 to 1 mm3 can be thus be achieved even with this method.
Furthermore three-dimensional image data sets are segmented as a rule after recording and reconstruction, with the data set being divided up into segments, i.e. volume areas, which are each assigned to the vessel structure or to the background. The image intensity of the background is set to zero. This is used to show the vessels without surrounding tissue and bone.
In the area of the neck of an aneurysm in particular it is very difficult to perform a segmentation correctly. Once again the lack of local resolution in the reconstructed 3D image data set as well as reconstruction artifacts are responsible for this. If the segmentation is thus optimized to a specific area (e.g. the aneurysm neck), under some circumstances this leads to a less than optimum segmentation of other areas (e.g. feeding vessels, middle of the aneurysm).
What is referred to as Digital Subtraction Angiography (DSA) is also known for showing vessels. In this case two chronologically consecutive images of the vessel structure are recorded, usually with a C-arm x-ray device. A contrast medium is injected into the bloodstream between the images. The two x-ray images thus only differ in the depiction of the vessels which are hardly visible in the first image (mask image) but which are strongly contrasted in the second image (filling image) however. The digitized images are subtracted from one another. Thus only the contrasted blood vessels are to be seen in the difference image, the DSA. The DSA thus delivers two-dimensional (2D) images with a high local resolution, but without depth information. Thus the DSA is also referred to below as the “2D DSA”.
WO 2004/072903 A2 discloses a method to creating a 3D model of a vessel structure, which also uses a reconstructed three-dimensional image and 2D projection images. In this patent the center lines of the vessels are first selected on the 3D image and then automatically segmented. These center lines are projected onto the 2D projection images and the outlines of the vessel structure on the 2D projection images is determined and projected back into the 3D image.